The invention relates to the so-called optical disc record system. In this system a spiral groove or track on a record disc is scanned by an optical pick up which can reproduce the track modulation. Such a system is disclosed in U.S. Pat. No. 4,065,786 which shows how a tracking error and recorded signal information can be obtained from a four quadrant photo detector. An optical system directs a coherent blue light source into a beam that impinges upon the record track. The photodetector is illuminated by the light reflected from the track. The circuits that combine the photodetector outputs to produce the signal and tracking error components are referred to as matrix circuit elements. The tracking error signal is employed to operate a servo system that deflects the light beam to its desired location. When the beam is on track the signal output is a reproduction of the track modulation.
The system disclosed in U.S. Pat. No. 4,065,786 employs a single optical beam which takes the place of the well known phonograph pick up stylus. This results in a pick up having reduced mechanical resonances and eliminates the friction that ordinarily produces wear in the pick up and the disc. Since the coherent light can be focused to a very small spot, the track can be recorded in a relatively small space. Other optical disc pick up systems employ plural optical beams, the most common being a three-beam system. In the three-beam system a six-element photodetector is employed to develop the recorded signal and tracking error information. In either a three-beam or a one-beam system it is desirable to develop, within the matrix, a third error signal that is related to beam focusing. This signal will be employed to drive a servo system that positions the focusing lens with respect to the disc surface. It would be desirable to have an integrated circuit (IC) that can be employed in either of the above-described systems with a minimum of redundancy and complexity.